1. Field of the Invention
This invention relates to a process for the production of hard plastic foams, using a polyvinylchloride base, from a mixture of polyvinylchloride, temporary softeners and foaming agents, wherein the mixture is heated, cooled and expanded.
2. Prior Art
It is known to produce hard plastic foams from polyvinylchloride. In such case, a mixture of polyvinylchloride, foaming agent and solvent, such as acetone or toluene, which serves as a temporary softener, is molded in compression molds or processed by extrusion or injection molding and is then expanded. After complete expansion the solvent must be removed by evaporation. Such a processing step is very expensive and harmful to the environment.
It is also known to use monomers, such as, isocyanates, diisocyanates or methylmethacrylates, as temporary softeners. (See Benning, "Plastic Foams", Wiley-Interscience 1969, pp. 112 and 115.) Such monomers are converted into hard polymeric products during the heating period and/or during the expansion step, under the influence of water (in the case of isocyanate and diisoycanate) or using radial initiators in the case of methylmethacrylates.
When methylmethacrylate is used, it is difficult to achieve good uniform and sufficient expansion, as a result of the polymerization which starts under the effect of heat. The monomer is polymerized too quickly, so the softening effect, which is needed for a good expansion, is lost too quickly.